Temperature sensor using an optical fiber

ABSTRACT

A temperature sensor that has an elongated sensing element having a length of at least 10 m, measured at a temperature of 20° C. The elongated sensing element includes an elongated jacket and an optical fiber mounted in the jacket and having an EFL of at least 0.35%, wherein the elongated sensing element has an average temperature error of less than 2° C.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 USC 119(e) of U.S.Provisional Patent Application No. 60/986,766 filed on Nov. 9, 2007 byBai Zhou et al. and hereby incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates to temperature sensing, in particular to atemperature sensor that can measure temperatures by using opticalproperties of an optical fiber. The invention finds applications in themining, gas and oil exploration/production industry, in particular theSteam Assisted Gravity Drainage (SAGD) and other heated oil wells orpipelines, serpentine or path probes in various chemical or physicalindustrial processes, Electric Submersible Pumps (ESP) used for oilrecovery or other applications, transformers, heat exchangers, boilers,separation unitary processors, reactors, heat distribution networks,skin probes, thermo-cartography applications, thermo-alarm networks andthermo-aging monitoring applications, among others.

BACKGROUND OF THE INVENTION

The gas and oil exploration or production processes, in particular theSAGD processes often require a precise knowledge of the temperaturesthat exist in an oil well. In light of the extremely flammable nature ofthe environment existing in a gas/oil well, optical fibers are used toprovide temperature measurements. Typically, an optical fiber basedtemperature sensor has an optical fiber that establishes an opticalpath. Gratings are formed at spaced apart intervals along that path.When the optical fiber undergoes expansion or contraction resulting fromtemperature changes, the optical properties of the gratings are altered.The optical properties can be measured by sending in the optical fiberan interrogation signal and then reading the responses of the individualgratings. A temperature measurement can be derived on the basis of thoseresponses.

A challenge in this type of measurement setups is to lay the opticalfiber in the well in a way to minimize any tension or bending strains.Since a tension will have the same effect on a grating as temperaturedoes, namely alter the optical properties of the grating, any artificialtension that is not due to temperature will induce an error in thetemperature measurement.

Accordingly, there is a need in the industry to develop new sensors fortemperature measurements that use optical fibers and that are protectedagainst deleterious effects of tension/bending strains that may ariseduring installation of the sensor or during use thereof.

SUMMARY OF THE INVENTION

As embodied and broadly described herein, the invention provides atemperature sensor. The temperature sensor comprises an elongatedsensing element having a length of at least 10 m, measured at atemperature of 20° C. The elongated sensing element includes anelongated jacket and an optical fiber mounted in the jacket and havingan EFL of at least 0.35%. The elongated sensing element has an averagetemperature error of less than 2° C.

As embodied and broadly described herein, the invention also provides atemperature measurement system. The temperature measurement systemcomprises an elongated sensing element having a length of at least 10 m,measured at a temperature of 20° C. The elongated sensing elementincludes an elongated jacket and an optical fiber mounted in the jacketand having an EFL of at least 0.35%. The elongated sensing element hasan average temperature error of less than 2° C. The optical fiberdefines an optical path for conveying an optical signal, the opticalpath manifesting an interaction with the optical signal, the interactionproducing a measurable response, the response conveying informationabout temperature to which the optical fiber is exposed. The temperaturemeasurement system also comprises a measurement component coupled to theoptical fiber to sense the response and for deriving from the responseinformation on the temperature to which the optical fiber is exposed.

As embodied and broadly described herein, the invention also provides atemperature sensor. The temperature sensor comprises an elongatedsensing element having a length of at least 10 m, measured at atemperature of 20° C. The elongated sensing element includes anelongated jacket and an optical fiber mounted in the jacket and havingan EFL of at least 0.35%. The elongated sensing element has an S valueof at least about 1 mm2 per percent of EFL.

These and other aspects of the invention will become apparent to thoseof ordinary skill in the art upon review of the following description ofembodiments of the invention in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A detailed description of examples of implementation of the presentinvention is provided hereinbelow with reference to the followingdrawings, in which:

FIG. 1 is a schematical representation of a well drilled during anoil/gas exploration/production operation in which a temperature sensorconstructed according to a non-limiting example of implementation of theinvention can be used;

FIG. 2 is an enlarged partial longitudinal cross-sectional view of thetemperature sensor shown in FIG. 1; and

FIG. 3 is a cross-sectional view of the temperature sensor shown in FIG.2.

In the drawings, embodiments of the invention are illustrated by way ofexample. It is to be expressly understood that the description anddrawings are only for purposes of illustration and as an aid tounderstanding, and are not intended to be a definition of the limits ofthe invention.

DETAILED DESCRIPTION

A specific example of implementation will now be provided. This exampleis made in the context of a Steam Assisted Gravity Drainage (SAGD)production facility. However, it should be clearly understood that thepresent invention is not limited to this application only and can beused in many other different industries.

FIG. 1 is a schematic illustration of a SAGD oil production facility.The SAGD process is a thermal heavy oil recovery process. Wells 101 and102 are drilled in the ground in pairs. The wells are horizontallydrilled. The upper well 101 in a pair is the injection well while thelower well 102 in a pair is the production well.

The oil recovery process begins by injecting steam in both wells 101 and102 until the bitumen in the area surrounding the well pair is heatedenough to flow to the bottom production well 102. Steam circulationcontinues in the injection well 101 causing the fluid bitumen to flow tothe production well 102 by gravity. The liquefied bitumen is recoveredfrom the production well 101 by using any appropriate method such as bya pump.

The SAGD process requires monitoring of the temperature in the wellpairs. To provide such temperature measurements, a temperature sensor20, as shown in FIG. 2, can be used. The temperature sensor 20 is basedon an optical fiber and can provide measurements over long spans.Specifically, the temperature sensor 20 has a sensing unit 24 from whichextends an optical fiber length 22 leading to a measurement unit 26. Thesensing unit 24 is the portion of the temperature sensor 20 that picksup temperature information. It should be noted that the temperaturesensor 20 is designed to provide temperature information oversignificant spans allowing measuring the temperature over significantdistances. The length of the span over which the temperature is measureddepends on the length of the sensing unit 24.

In this specific example of implementation, the sensing element 24 canbe inserted in the production well 102 or the injection well 101. Inother examples, the sensing element 24 can be inserted in the productionwell 102 and another sensing element similar to the sensing element 24can be inserted in the injection well 101. The sensing element 24preferably should be long enough to run along a substantial portion ofthe well length, say at least 50%, preferably 75% and most preferably atleast 90% of the well length. The sensing element 24 can have a lengthof at least about 10 m. For typical SAGD applications, the sensingelement 24 would be in the order of 1 km and even more. For otherapplications, the sensing element 24 can be shorter. Generally, sensingelement lengths of about 10 m, 100 m, 500 m or 1 km and more arepossible.

The sensing unit 24 includes a jacket 28 that extends the full length ofthe sensing unit 24. The jacket 28 is provided to protect the opticalfiber 30 placed in the jacket 28. The jacket 28 should be flexibleenough to allow the sensing unit 24 to bend during installation and atthe same time it should have a sufficient crush resistance to preventdamage to the optical fiber 30 that may arise as a result of interactionwith adjoining components or pressure arising within the well. A widerange of materials can be selected for the manufacture of the jacket 28.Metallic materials, such as stainless steel, are a good choice sincethey readily offer the flexibility and crush resistance criteria. Thechoice of the specific metallic material will be governed by thespecific conditions that exist in the environment in which the sensingunit 24 is to be placed. In corrosive environments, stainless steel isusually a good choice. Polymeric, ceramic or composite materials canalso be used provided that they have the requisite properties.

A wide range of different techniques can be used to make the jacket 28.When the jacket is made of metallic material it can be manufactured as atube and the optical fiber 30 inserted through the tube. This would workfor very short sensing units 24. Alternatively, the jacket 28 can beformed in a continuous process around the optical fiber 30. Morespecifically, the jacket is made from a strip of metallic materialsupplied from a coil. The optical fiber 30 is run parallel to the stripwhile the strip is fed to a forming station that uses a suitable die toprogressively bend the strip into a tube around the optical fiber 30. Inthis fashion, the longitudinal edges of the strip are brought in a faceto face relationship, the resulting tube enclosing the optical fiber 30.To seal the tube the longitudinal edges are welded.

Since this is a continuous process, a sensing unit 24 of any length canbe made as long as a metallic material strip and an optical fiber 30 ofthe proper length dimensions can be provided.

Once the tube portion of the jacket 28 is formed around the opticalfiber 30, the end portion 32 of the jacket 28 can be closed. A suitablecap 34 can be welded or inserted within or outside the jacket 28. Asimilar cap 36 is also provided at the opposite end of the jacket 28,with the exception that the cap 36 has a passageway 38 through which theoptical fiber 30 can pass. Alternatively, the jacket 28 may be left openat the ends.

When the jacket 28 is made from polymeric or ceramic materials it can beextruded in the form of a tube directly around the optical fiber 30. Inthis case the polymeric or ceramic material is extruded through atube-shaped die while the optical fiber 30 is fed through an opening inthe die.

In the case of composite materials, fibers may be wrapped around theoptical fiber 30, again as a continuous process to build up a tube thatwill ultimately form the jacket 28.

The optical fiber 30 placed in the jacket 28 provides temperaturesensing over the length of the optical fiber 30. The temperature sensingmay be made at discrete areas along the optical fiber 30. In thisfashion, the sensing unit 24 generates temperature information atvarious discrete locations along its length. The spacing between thoselocations can be varied depending on the intended application. In aspecific and non-limiting example of implementation, the optical fiber30 includes discrete sensors at each measurement location which can beinterrogated independently of one another. One possible example of adiscrete sensor is a grating, such as a Fiber Bragg grating (FBG).

The sensing mechanism most often used in FBGs arises from the fact thatthe reflection wavelength for the forward propagating core mode varieslinearly with temperature and strain. Since the wavelength can bemeasured with an accuracy of 10 pm relatively easily near 1550 nm, thisrepresents a relative resolution of about 6 ppm. A variant of the sameconcept uses so-called Long Period Gratings (LPG) where coupling occursbetween the forward propagating core mode and forward propagatingcladding modes. In this case, the sensitivity of the resonancewavelength to perturbations can be greatly enhanced for some of thecladding modes.

Another possible example of a discrete sensing unit is a Fabri-Perotcavity.

In a possible variant, the sensing of the strain induced in the opticalfiber 30 as a result of temperature changes is made by measuring theback scattered light produced as the optical interrogation signalpropagates along the optical fiber 30. Without intent of being bound byany particular theory, scattering in general and back scattering inparticular arises as a result of inhomogeneities in the refractive indexin the optical path or due to acoustic waves known as phonons. Differentcomponents of the back-scattered light can be identified, such asRaleigh, Raman and Brillouin scattering. The Brillouin scatteringinduces a Doppler frequency shift of the scattered light. This isusually referred to as the spontaneous Brillouin scattering.

For instance an interrogation signal in the form of a pulse willpropagate along the optical fiber 30 and as it propagates it willinteract with the optical path. This interaction produces a measurableback scattering response. The back scattering response propagates in anopposite direction with relation to the direction of travel of theoptical interrogation pulse. Generally the response is downshifted infrequency relative to the frequency of the optical interrogation pulsewhich allows distinguishing the response from the optical interrogationpulse itself.

When the optical fiber 30 is at the same temperature along its length,the back scattering phenomenon is homogeneous along the length of theoptical fiber 30. In other words, as the optical interrogation pulsetravels along the optical fiber, the interaction with the optical pathremains the same. Accordingly, the frequency shift between the responseand the optical interrogation signal does not change.

In the case when strain is induced in the optical fiber 30, which canarise if the ambient temperature changes, the interaction will alsochange. The temperature change produces alterations in the optical pathand those alterations affect the interaction between the optical pathand the optical interrogation pulse. Such interaction changes manifestthemselves as frequency shifts of the response. Accordingly thefrequency shift between the response and the frequency of the opticalinterrogation pulse constitutes an indicator of the temperature changein the optical fiber 30.

It should be noted that the interaction between the opticalinterrogation signal and the optical path occurs in a continuous fashionas the optical interrogation signal propagates along the optical fiber30. This is to be distinguished from the previous examples where theinteraction is of discrete nature and occurs only at specific locationsin the optical fiber where sensors, such as gratings are placed.Accordingly, when the optical interrogation signal propagates along theoptical fiber 30 it produces a response only when it encounters agrating. No response is produced between gratings.

By using a continuous interaction system of the type described earlierthere is no necessity to provide any sensors in the optical fiber 12. Infact, the optical fiber 12 is a standard optical fiber without anymodifications or changes required.

The measuring unit 26 is provided to produce the interrogation pulse,read the response produced by the optical fiber 30 and then derive fromthose responses temperature information. The measuring unit 26 includesseveral components which in practice will be controlled via a computer.

The structure and functionality of the measuring unit 26 will obviouslybe dependent on the type of sensing mechanism used by the sensing unit24. When discrete sensors are used, the interrogation signal can bedesigned with a wavelength to encompass the entire reflection wavelengthrange of the gratings array in the optical fiber 30. Accordingly, asingle interrogation signal will prompt all the gratings to respond. Theresponses can be distinguished from one another on the basis of theirwavelength. Since the position of each grating in the optical fiber 30is known, then it is possible to derive on the basis of the reflectionwavelength shift the temperature in the area where the grating islocated. Since each grating provides a temperature measurement, thecollection of temperature measurements can be used to create atemperature profile along the sensing unit 24.

In the case of backscattering, the measuring unit 26 includes aninterrogation source that generates the optical interrogation signal.The interrogation source can be a laser to deliver an opticalinterrogation signal in the form of a pulse. The duration, intensity andwavelength (frequency) of the pulse can be determined according to theintended application.

A response sensor is connected to the optical fiber 30 to sense theresponse produced by the optical interrogation signal. The responsesensor detects the presence of the response and also determines thewavelength (frequency) of the response.

A processing component receives the wavelength information from theresponse sensor. Specifically, the processing component includes atiming unit. The timing unit drives the interrogation source. When theinterrogation source is triggered by the timing unit an opticalinterrogation pulse is injected into the optical fiber 30. At the sametime a high precision timing circuit is triggered to count time. Sincethe travel speed of the optical interrogation pulse in the optical fiber30 is known and the speed of travel of the response is also known, it ispossible to determine, on the basis of the time span between the triggerof the optical interrogation pulse and the reading of the response thearea of the optical path (the distance from the extremity of the opticalfiber 30 at which the optical interrogation pulse in injected and wherethe response is read) that has produced the response.

For instance if it is desired to read the response produced by the areaof the optical fiber 30 that is 500 m from the extremity of the opticalfiber 30, the timing unit counts time that corresponds to the timenecessary for the optical interrogation pulse to travel 500 m down theoptical fiber 30, plus the time it takes the response to travel back the500 m distance to the extremity of the optical fiber 30. As indicatedearlier, since the speed of the travel of the optical interrogationpulse and of the response are known, it is possible to compute theduration of the time interval necessary to get a reading from a desiredlocation on the optical fiber 30.

Once the time duration computed by the timing unit has elapsed, thetiming unit signals the response sensor to take a reading of thewavelength of the response. The wavelength information captured by theresponse sensor at that particular time indicates the intensity of thetemperature induced strain acting at the location of the optical fiber30 where the measurement is being read. The position of that location,in terms of distance measured along the optical fiber 30 is determinedon the basis of the time interval between the triggering of the opticalinterrogation pulse and the wavelength reading.

The same operation can be repeated to measure the strain induced on theoptical fiber 30 but at a different position, by changing the timeinterval. This can be done by triggering a new optical interrogationpulse and extending the time interval in order to obtain a readingfurther down the optical fiber 30. The different data points obtained inthis fashion can be used to create a temperature profile for the opticalfiber 30.

The resolution of the temperature profile, in other words, the minimaldistance between measurement points along the optical fiber 30, dependslargely on the accuracy of the timing unit. With a highly accuratetiming unit, of a type that is commercially available, it is possible toread the strains at steps as low as 25 cm.

In a possible variant, a pump source is used that allows creating aStimulated Brillouin Scattering (SBS) interaction. The pump sourceproduces a laser beam that is introduced into the optical fiber 30. Ifthe intensity of the beam is sufficiently high its electric field willgenerate acoustic vibrations in the optical path via electrostriction.This can generate Brillouin scattering that can be effectively amplifiedby injecting in the optical fiber 30 an optical interrogation pulse. TheSBS is advantageous in that it produces a stronger response that iseasier to pick up and process.

With specific reference to FIGS. 2 and 3, the optical fiber 30 is laidloosely in the jacket 28 such as to allow the optical fiber 30 and thejacket 28 to expand or contract, as a result of temperature variations,independently of one another. In the examples described earlier, thejacket 28 is made of materials that have a Coefficient of ThermalExpansion (CTE) that is greater than the CTE of the optical fiber 30. Asa result, the jacket 28 expands more than the optical fiber 30 when thetemperature in the area in which the sensing unit 24 is placedincreases. For example, the CTE ratio (CTEjacket/CTEfiber) is of atleast about 1.0001, preferably of at least about 10 and most preferablyof at least about 20.

For instance, in the case of a sensing unit 24 having a length of 1.5 kmand with a jacket made of stainless steel, the jacket will expand byabout 3 m to 4 m, when the sensing unit 24 is exposed to temperatures inthe range from 200° C. to 250° C., by comparison to the jacket length atthe ambient temperature (20° C.). At the same temperature shift, theoptical fiber 30 would expand by about 0.1 m to 0.15 m. Note that thetemperature range of about 200° C. to about 250° C. typicallycorresponds to the temperatures encountered in a well in a SAGDoperation.

To accommodate this difference, Excess Fiber Length (EFL) is provided inthe jacket 28. This means that at the temperature at which the EFL ismeasured, which typically would be the ambient temperature, the lengthof the optical fiber 30 will exceed the length of the jacket 28. Thedegree of EFL is such that once the jacket 28 is within its operationaltemperature range, in other words the temperature range in which thetemperature measurements are to be made, the length of the optical fiber30 will be about the same as the length of the jacket 28. The amount ofEFL to be built into the sensing unit 24 depends largely on the intendedapplication. Applications where high temperatures are to be measuredwould require more EFL than those where lower temperatures are measured.

The amount of EFL built into the sensing unit 24 can be expressed interms of percentage. Specifically, it can be computed by

$\frac{\left( {F_{l} - J_{l}} \right) \times 100\%}{F_{l}}$

at an ambient temperature which for the purpose of this specification isdefined to be 20° C. In the above equation, F1 is the total opticalfiber length while J1 is the total jacket 28 length. For instance, forthe temperature sensor 20 considered in this example, the EFL may be ofat least about 0.35% (0.0035), preferably at least about 0.50%, and mostpreferably at least about 0.70%.

The desired amount of EFL is normally provided at the manufacturingstage by stretching the jacket 28 while the optical fiber 30 is beingplaced into it. Specifically, this is performed by the mechanism whichforms the metallic strip as a tube around the optical fiber 30. Onepossibility is to feed the metallic strip between series of spaced apartrollers that turn at slightly different speed such as to create astretch into the strip. Once the tube is formed, the stretch pressure isreleased and the tube then resiliently shrinks, thus causing the EFL toarise.

Since the sensing unit 24 is intended to operate within a range oftemperatures where accurate temperature measurements are expected to bemade, the jacket 28 and the optical fiber 30 will experience a certaindegree of relative motion as the temperature outside the sensing unit 24varies within that range. For instance consider the situation where theoutside temperature (the temperature outside the sensing unit 24) isnear the lower end of the operational temperature range. When theoutside temperature increases the jacket 28 will elongate more than theoptical fiber 30 and this additional elongation will translate as arelative sliding movement between the two components. If the temperaturenow decreases toward the starting point, the reverse movement willoccur.

The relationship between the jacket 28 and the optical fiber 30 is suchthat the movement of both components as a result of temperature changesreduces as much as possible the occurrence of residual stresses on theoptical fiber 30. Obviously, such residual stresses are undesirable. Ifthe jacket 28 would be allowed during thermal expansion to drag theoptical fiber 30 and somewhat stretch it, the resulting stress in theoptical fiber 30 will register as a temperature change, thus causing ameasurement error.

To reduce the interaction between the jacket 28 and the optical fiber30, one possibility is to make the jacket 28 large enough such that theoptical fiber 30 virtually free floats inside and both components canmove without causing undue stresses on the optical fiber 30. The degreeof spacing that should be provided between the optical fiber 30 and thejacket 28 is dependent on the degree of EFL built into the sensing unit24; the larger the EFL the higher the spacing should be. Therefore thespacing S is defined by

$\frac{A_{j}}{EFL}$

where Aj is the cross-sectional area of the jacket 28 and EFL isexpressed as a percentage. Aj is assessed by measuring or computing theinternal cross-sectional area of the jacket 28 at every meter of thejacket length, at 20° C., and the results are averaged. In the exampleshown in FIG. 3, where the jacket 28 has a circular cross-sectionalshape, the internal radius r of the jacket 28 is used to compute thecross-sectional area by the well known formula πr². Since thecross-sectional area is the same along the entire length of the jacket28, a single measurement will suffice and there is no need to makemultiple measurements.

According to one aspect of the invention, the sensing unit 24 has an Svalue of at least about 1 mm2 per percent of EFL, preferably of at leastabout 6.25 mm2 per percent of EFL and most preferably of at least about10 mm2 per percent of EFL.

Other possibilities to reduce the interaction between the jacket 28 andthe optical fiber 30 is to coat the internal surface of the jacket 28and/or the outside surface of the optical fiber 30 with frictionreducing material such as to facilitate the relative sliding movementbetween the two components. An example of such friction reducingmaterial is polytetrafluoroethylene. Another possibility is to fill thejacket 28 with a suitable liquid to provide lubrication at the contactsurfaces between the jacket 28 and the optical fiber 30. Evidently, thechoice of the liquid will depend on the operational temperature range.For instance, in the example of an operational temperature range from200° C. to 250° C., a suitable lubricant working well in thattemperature range can be used.

The approaches described above can be used individually or incombination to reduce the jacket 28/optical fiber 30 interaction.

The sensing unit 24 is characterized by a maximal average temperatureerror D. The average temperature error D for a sensing unit isdetermined by the following procedure:

A sensing unit is provided and it is laid flat on a table or any othersuitable support. The arrangement is such that the sensing unit remainsstraight and horizontal during the entire testing procedure. The sensingunit has a jacket in which is placed at least one optical fiber runninglengthwise of the jacket. The optical fiber has an EFL of at least 0.35%measured at 20° C. The sensing unit that is being tested should have alength of at least 10 m. If the sensing unit is too long and it is notpractical to build a test set up to support it straightened and in ahorizontal position, then a ten meter sample should be cut ormanufactured separately and that sample is tested.

The optical fiber has at least one temperature measuring point per 10 mof length of the sensing unit.

The sensing unit is laid flat and the ambient temperature raised to theminimal temperature of the expected operational temperature range inwhich the sensing unit would be used in operation. For example, in theexample considered previously herein, this temperature is 200° C.Heating of the sensing unit is to be effected uniformly over its length.The sensing unit is left at that temperature for 10 minutes and atemperature reading taken via the optical fiber. At the same time anindependent temperature measurement is obtained from a calibrated probeat the point along the length of the sensing unit 24 which correspondsto the location where the temperature is read by the optical fiber 30.

The temperature is raised by 10° C. and the process described earlier isrepeated to obtain a temperature measurement from the sensor unit and acorresponding temperature measurement from the calibrated probe.Temperature measurements are thus obtained for every 10° C. intervalfrom the lower end of the operational temperature range (200° C.) up tothe upper end of the range, which is 250° C. in this example.

Once the upper end temperature has been reached, the temperature iscycled down, and measurements made, as described earlier at each 10° C.step, until the lower end of the temperature range has been reached.

The measurements provide a set of data point pairs. There is a datapoint pair associated for each measuring point, for each 10° C. intervaland for each cycle (up or down). Each data point pair includes atemperature measurement obtained via the optical fiber and one obtainedvia the calibrated probe. For instance if there are two measurementlocations on the sensing unit 24, say one measurement location at 5 mfrom one extremity of the sensing unit and another measurement locationat 5 m from the other extremity of the sensing unit, and the temperaturerange is of 200° C. to 250° C., then a total of 24 data point pairs willbe obtained. Those break down as follows:

There are 12 point pairs per measurement location.

Per measurement location, there are 6 point pairs, one pair for each 10°C. spread when the temperature is cycled upward (point pairs at 200,210, 220, 230, 240 and 250° C., respectively).

Per measurement location, there are 6 point pairs, one pair for each 10°C. spread when the temperature is cycled downward (point pairs at 250,240, 230, 220, 210 and 200° C., respectively).

For each data point pair a temperature error is computed. This is doneby subtracting one temperature measurement from the other and thentaking the absolute value of the result.

The temperature errors are summed and the result is divided by thenumber of temperature error values to obtain an average temperatureerror. The average temperature error is then divided by the length ofthe sensing unit to obtain the average temperature error per unitlength.

According to one aspect of the invention, the maximal averagetemperature error D is of less than 2° C., preferably of less than 1° C.and most preferably of less than 0.2° C.

Although various embodiments have been illustrated, this was for thepurpose of describing, but not limiting, the invention. Variousmodifications will become apparent to those skilled in the art and arewithin the scope of this invention, which is defined more particularlyby the attached claims.

1. A temperature sensor, comprising: an elongated sensing element havinga length of at least 10 m, measured at a temperature of 20° C., theelongated sensing element including: an elongated jacket; an opticalfiber mounted in the jacket and having an EFL of at least 0.35%; and theelongated sensing element having an average temperature error of lessthan 2° C.
 2. A temperature sensor as defined in claim 1, wherein theelongated sensing element has an average temperature error of less than1° C.
 3. A temperature sensor as defined in claim 2, wherein theelongated sensing element has an average temperature error of less than0.2° C.
 4. A temperature sensor as defined in claim 1, wherein theoptical fiber has an EFL of at least 0.50%.
 5. A temperature sensor asdefined in claim 4, wherein the optical fiber has an EFL of at least0.70%.
 6. A temperature sensor as defined in claim 1, wherein theelongated sensing element has a length of not less than 100 m.
 7. Atemperature sensor as defined in claim 6, wherein the elongated sensingelement has a length of not less than 500 m.
 8. A temperature sensor asdefined in claim 7, wherein the elongated sensing element has a lengthof not less than about 1 km.
 9. A temperature sensor as defined in claim1, wherein the jacket is characterized by a CTEjacket, the optical fiberis characterized by a CTEfiber, and a ratio CTEjacket/CTEfiber has avalue of at least about 1.0001.
 10. A temperature sensor as defined inclaim 9, wherein the ratio CTEjacket/CTEfiber has a value of at leastabout
 10. 11. A temperature sensor as defined in claim 10, wherein theratio CTEjacket/CTEfiber has a value of at least about
 20. 12. Atemperature measurement system, comprising: an elongated sensing elementhaving a length of at least 10 m, measured at a temperature of 20° C.,the elongated sensing element including: an elongated jacket; an opticalfiber mounted in the jacket and having an EFL of at least 0.35%; theelongated sensing element having an average temperature error of lessthan 2° C.; the optical fiber defining an optical path for conveying anoptical signal, the optical path manifesting an interaction with theoptical signal, the interaction producing a measurable response, theresponse conveying information about temperature to which the opticalfiber is exposed; and a measurement component coupled to the opticalfiber to sense the response and for deriving from the responseinformation on the temperature to which the optical fiber is exposed.13. A temperature measurement system as defined in claim 12, wherein theelongated sensing element has an average temperature error of less than1° C.
 14. A temperature measurement system as defined in claim 13,wherein the elongated sensing element has an average temperature errorof less than 0.2° C.
 15. A temperature measurement system as defined inclaim 12, wherein the optical fiber has an EFL of at least 0.50%.
 16. Atemperature measurement system as defined in claim 15, wherein theoptical fiber has an EFL of at least 0.70%.
 17. A temperaturemeasurement system as defined in claim 12, wherein the elongated sensingelement has a length of not less than 100 m.
 18. A temperaturemeasurement system as defined in claim 17, wherein the elongated sensingelement has a length of not less than 500 m.
 19. A temperaturemeasurement system as defined in claim 18, wherein the elongated sensingelement has a length of not less than about 1 km.
 20. A temperaturemeasurement system as defined in claim 12, wherein the jacket ischaracterized by a CTEjacket, the optical fiber is characterized by aCTEfiber, a ratio CTEjacket/CTEfiber has a value of at least about1.0001.
 21. A temperature measurement system as defined in claim 20,wherein the ratio CTEjacket/CTEfiber has a value of at least about 10.22. A temperature measurement system as defined in claim 21, wherein theratio CTEjacket/CTEfiber has a value of at least about
 20. 23. Atemperature measurement system as defined in claim 12, wherein themeasurement component is operative for deriving from the responseinformation on the temperature to which the optical fiber is exposed ata plurality of spaced apart locations along the length of the sensingelement.
 24. A temperature measurement system as defined in claim 23,wherein the optical fiber includes a plurality of spaced apart gratingshaving a temperature dependant response to optical interrogation.
 25. Atemperature measurement system as defined in claim 23, wherein theinteraction is a continuous interaction that occurs along the opticalpath, wherein the interaction produced back-scattering, the measurementcomponent sensing the back-scattering.
 26. A temperature sensor,comprising: an elongated sensing element having a length of at least 10m, measured at a temperature of 20° C., the elongated sensing elementincluding: an elongated jacket; an optical fiber mounted in the jacketand having an EFL of at least 0.35%; and the elongated sensing elementhaving an S value of at least about 1 mm2 per percent of EFL.
 27. Atemperature sensor as defined in claim 26, wherein the elongated sensingelement has an S value of at least about 6.25 mm2 per percent of EFL.28. A temperature sensor as defined in claim 27, wherein the elongatedsensing element has an S value of at least about 10 mm2 per percent ofEFL.
 29. A temperature sensor as defined in claim 26, wherein theoptical fiber has an EFL of at least 0.50%.
 30. A temperature sensor asdefined in claim 29, wherein the optical fiber has an EFL of at least0.70%.
 31. A temperature sensor as defined in claim 26, wherein theelongated sensing element has a length of not less than 100 m.
 32. Atemperature sensor as defined in claim 31, wherein the elongated sensingelement has a length of not less than 500 m.
 33. A temperature sensor asdefined in claim 32, wherein the elongated sensing element has a lengthof not less than about 1 km.
 34. A temperature sensor as defined inclaim 26, wherein the jacket is characterized by a CTEjacket, theoptical fiber is characterized by a CTEfiber, and a ratioCTEjacket/CTEfiber has a value of at least about 1.0001.
 35. Atemperature sensor as defined in claim 34, wherein the ratioCTEjacket/CTEfiber has a value of at least about
 10. 36. A temperaturesensor as defined in claim 35, wherein the ratio CTEjacket/CTEfiber hasa value of at least about 20.